MacConkey Medium

Issues of Concern

Key components of the MacConkey medium include crystal violet dye, bile salts, lactose, and neutral red (pH indicator). Crystal violet dye and bile salts halt the growth of gram-positive bacteria. This allows only gram-negative species to form colonies on MAC agar.[2] MacConkey agar contains the essential nutrients required for microorganism growth. Additional key components include crystal violet dye, bile salts, lactose, and neutral red (a pH indicator). The lactose in the agar is a source of fermentation. Lactose-fermenting microorganisms will produce organic acids, particularly lactic acid, which will lower the pH. Neutral red is a pH indicator that turns from off-white to bright red/pink as the pH drops below 6.8.

Different species will yield colonies in varying appearances on a MacConkey medium based on their ability to ferment lactose. This gives McConkey agar its differentiating property. 

• Lactose (Lac) positive (pink colonies):
• Lactose-fermenting species will grow pink colonies. Lactose fermentation produces acidic byproducts that lower the pH, turning the pH indicator pink.
• Examples of Lac-positive species: Escherichia coli, Enterobacteria, Klebsiella 
• Lac negative (white colonies)
• Gram-negative bacterial species will still form colonies, but colonies will have a white appearance as there will be no change in pH in the absence of lactose fermentation.
• Examples of Lac-negative species: Salmonella, Proteus, Yersinia, Pseudomonas
• No colonies:
• Gram-positive bacteria will not form any colonies on MacConkey medium. 
• Slow:
• Weak lactose fermenters will form colonies slower than the rest.  
• Examples of slow lac fermenters: Serratia, Citrobacter 
• Mucoid: (sticky, wet colonies)
• Encapsulated bacteria produce capsules using lactose. This gives sticky, wet-appearing colonies.
• Examples of mucoid colony-forming species: Klebsiella, enterobacter

Clinical Significance

A pure bacterial culture remains crucial to assess its virulence, its susceptibility to antibiotic therapy, and its genome sequence in order to facilitate the understanding and treatment of caused diseases. MAC's selective and differentiating properties as a culture medium allow isolating colonies of pure bacterial culture from a source sample. Typically in a clinical setting, the collected sample is placed on a panel of many growth media for identification and isolation of bacterial cultures.[3] MAC is one of the widely used growth media as it can work to selectively grow gram-negative bacteria and further differentiate them based on their fermentation profile. Many pathogenic gram-negative bacteria can be differentiated by MAC, especially bacterial gastroenteritis-causing species. The diagnostic potential is immense. MAC is essentially a versatile foundation, in which additional substrates, such as sorbitol, can be incorporated for further differentiation. Another example is the addition of antibiotics for testing drug resistance. In intensive care units, testing multi-drug resistance in gram-negative bacteria is an important surveillance measure.[4] Gram-negative enteric bacteria are a common cause of bacterial gastroenteritis, which is characterized by diarrhea, vomiting, and abdominal cramping. Escherichia coli and Campylobacter jejuni are some of the common causes of bacterial gastroenteritis. When bacterial gastroenteritis is suspected, the patient's specimen can be sampled and cultured on a panel of bacterial cultures, which includes MacConkey medium. MAC contributes to the identification of the causal agent by providing lactose-fermentation profiles in gram-negative species.[5]

Nursing, Allied Health, and Interprofessional Team Interventions

A pure bacterial culture remains essential for the study of its virulence, its antibiotic susceptibility, and its genome sequence in order to facilitate the understanding and treatment of caused diseases. Bacterial cultures are still widely used laboratory techniques that many physicians rely on to make their final diagnoses. Many healthcare professionals are involved in this process, and proper communication and coordination will enhance patient-centered care, improve outcomes and patient safety, and enhance team performance. Bacterial culture from a patient's specimen is a commonly ordered test that many physicians use to aid their medical decision-making process. Nurses are frequently involved in obtaining and transporting samples from the patient to the laboratory. Clinical microbiologists must be trained with a thorough understanding of the principles to run the tests, interpret the results, and report the findings. Sterile sampling techniques and proper handling of the specimen during handling and transport of samples are required to avoid contamination. Any healthcare professionals handling these patient samples must be adequately trained in order to avoid contamination of the samples or accidental exposure and spread of potential pathogens.[6][7]